1. Technical Field
The present invention relates to a semiconductor laser device and a fabrication method for the same, and more particularly to red and infrared semiconductor laser devices and fabrication methods for the same
2. Background Art
In these days, demands for a light source for a high-speed writable optical disk system, such as a recording 16x DVD having not only playback function but also recording function, have increasingly grown. For such a light source, required is a semiconductor laser device having at least 300 mW high-output operation capability.
In general, in high-output operation of a semiconductor laser device, the cavity end face from which laser light is extracted (front end face) is coated with a dielectric film having a low reflectance of 10% or less, and the cavity end face opposite to the front end face (rear end face) is coated with a dielectric film having a high reflectance of 85% or more. With such antireflection (AR) coating and high reflection (HR) coating, the external differential quantum efficiency (slope efficiency) in the current-light output characteristic can be improved. This permits high light output with a small current amount injected. Also, the power density of laser light at the front end face is reduced, and this can prevent occurrence of a catastrophic optical damage (COD) in which the laser end face is melted down with the light output of the laser light itself.
Having a low front-face reflectance and a high rear-face reflectance is effective in improvement of the COD level and the light extraction efficiency. However, if the reflectance of the front end face is too low, the laser light fed back inside the cavity is reduced, causing increase in oscillation threshold current. Also, with the reflectance of the front end face being low, when the semiconductor laser device is applied to an optical disk, noise is likely to occur due to return light reflected from the optical disk (return light-induced noise). In view of the above, in a high-output laser, the front end face is coated so as to have a reflectance of about 5 to 10% to ensure attainment of high light extraction efficiency and also reduction in return light-induced noise. The rear end face is generally coated so as to have a reflectance of about 95 to 100% to ensure as high reflectance as possible.
In a high-output semiconductor laser device, in which the reflectance is largely different between the front and rear end faces, light propagating in the active layer in the cavity direction exhibits asymmetric light distribution intensity: the light distribution intensity is higher on the front end face side than on the rear end face side. In this situation, more intense stimulated emission occurs on the front end face side high in light distribution intensity than on the rear end face side. Accordingly, on the front end face side, a larger number of electron-hole pairs must be injected in the active layer than on the rear end face side. This is likely to cause shortage of electron-hole pairs in the active layer on the front end face side especially during high-output operation. Such shortage of electron-hole pairs may become a cause of saturation of the luminous efficiency. In attainment of a high-output laser having an output of 200 mW to 300 mW or more, this may degrade the temperature characteristic resulting in serious obstruction.
A normal AlGaInP semiconductor laser widely adopts a GaAs substrate using a plane tilted from (100) plane by 7° to 15° in [011] direction as the principal plane, to obtain a good temperature characteristic. In formation of a cavity on such a substrate, if only chemical wet etching is employed to form a ridge stripe portion, the acute angles formed between the crystal plane and the sidewalls of the stripe portion will be θ1=54.7°−θ°, θ2=54.7°+θ°. If the ridge stripe portion is formed by a physical method such as ion beam etching, the cross-sectional shape of the stripe portion can be axis-symmetric with respect to the axis in the layered direction in the crystal cross section. In this case, however, physical damage may be left behind on the ridge sidewalls, causing a leak at the interface between the ridge sidewalls and a current narrowing layer and thus deteriorating the current narrowing effect. For this reason, when a physical method is employed to form the stripe portion, also, it is desired to wet-etch the sidewalls of the ridge a little before growth of the current narrowing layer. Therefore, in this case, also, the shape of the stripe portion is axis-asymmetric.
In an axis-asymmetric stripe portion, the distribution of light propagating in a waveguide is asymmetric with respect to the stripe. In a high light density portion, radiation recombination of carriers often occurs under stimulated emission. Hence, the shape of spatial hole burning of carriers also exhibits asymmetric distribution. This indicates that the effective refractive index distribution in the horizontal direction parallel to the active layer is laterally asymmetric. As a result, the light distribution is likely to move in a direction in which the gain becomes relatively higher, and this is likely to cause a kink that is a bend formed in the light output-current characteristic.
To suppress occurrence of a kink, the stripe may be narrowed to allow the current flow to be concentrated in the narrow stripe region. This makes it possible to reduce the size of a depression in the carrier distribution generated due to spatial hole burning of operating carriers in the active layer, and thus suppress occurrence of a kink in the light output-current characteristic until reaching a higher output. However, a narrowed stripe causes increase in operating voltage along with increase in element serial resistance, as well as degradation in temperature characteristic along with increase in power consumption.
To solve the above problem, proposed is a semiconductor laser device that is formed on a compound semiconductor substrate tilted from (001) plane in [110] direction and has a stripe portion as shown in FIG. 13 (see Japanese Laid-Open Patent Publication No. 2004-200507, for example).
As shown in FIG. 13, a stripe portion 200 includes a first region 200a having a fixed width located in the center of the cavity and second regions 200b having a gradually widened width located on both sides of the first region. A current block layer (not shown) is provided on the sidewalls of the stripe portion 200, and the refractive index of the current block layer is smaller than that of the stripe portion 200.
With the above configuration, the effective refractive index inside the stripe portion 200 is higher than that outside the stripe portion 200. The light distribution is therefore confined inside the stripe portion 200 to permit fundamental transverse mode oscillation. Also, since the current block layer is transparent to laser light, the waveguide loss can be reduced, permitting reduction in operating current value. Moreover, in the first region 200a in which the width of the stripe portion 200 is fixed, the relative luminous position of the light distribution with respect to the shape of the stripe portion is fixed, and this stabilizes the optical axis of a far-field pattern (FFP) of the laser. In the second regions 200b in which the width of the stripe portion changes, the series resistance (Rs) can be reduced because the top of the stripe portion is widened. As a result, in the semiconductor laser device, while fundamental transverse mode oscillation stable in the light axis of FFP is attained, the operating current value and also Rs can be reduced.
However, the conventional semiconductor laser device described above has the following problem. For future implementation of a high-output laser permitting an output of 300 mW or more during high-temperature operation, the temperature characteristic must be improved, and for this purpose, the cavity length must be as large as about 1500 μm or more. The external differential quantum efficiency in the light output-current characteristic is proportional to mirror loss/(mirror loss+waveguide loss). The magnitude of the mirror loss is inversely proportional to the cavity length. Accordingly, as the cavity is longer, the mirror loss is smaller, and thus the external differential quantum efficiency is more susceptible to the waveguide loss. In a semiconductor laser device having 7%-reflectance coating on the front end face and 95%-reflectance coating on the rear end face, the mirror loss will be 15.1 cm−1 if the cavity length is 900 μm and 9 cm−1 if the cavity length is 1500 μm. The waveguide loss of a normal high-output laser is 10 cm−1 or less. It has been clarified that the waveguide loss is more influential on the external differential quantum efficiency as the cavity length is larger. Light lost in the waveguide loss is converted to heat. Accordingly, the element heating is accelerated during high-temperature, high-output operation, causing degradation in heat saturation level.
In consideration of the above, in a semiconductor laser having a long cavity exceeding 1500 μm, reduction in waveguide loss is very important in improving the light output level at which heat saturation occurs at high temperature. For this reason, in a long cavity laser, the waveguide loss must be made as small as possible for improvement of the light output causing heat saturation.
The conventional semiconductor laser device described above has the first region small in stripe width and the second regions gradually changing in stripe width, for improvement of the kink level. Propagating light scatters from the ridge sidewalls of the second regions changing in stripe width, causing radiation loss. With such radiation loss, the waveguide loss increases by about several cm−1, causing a problem of reduction in external differential quantum efficiency by about 10%.